The present invention relates in general to a power steering system for vehicles and particularly to a power steering system of the kind having a double-acting hydraulic cylinder which is operated to produce a steering assist force in accordance with a steering force applied to a steering wheel.
An example of a power steering system of the above described kind is disclosed in Japanese Patent Provisional Publication No. 59-118569. Another example is disclosed in Japanese Patent Provisional Publication No. 57-201767.
It is an object of the present invention to provide a power steering system which provides a steering feel which is not deteriorated by a road surface input, i.e., a sudden disturbance such as kickback applied to the system from the road surface.
It is a further object of the present invention to provide a power steering system of the foregoing character which can be operated manually without requiring an excessively large steering force when an oil pump is inoperative and is therefore highly useful from a safety driving point of view.
It is a further object of the present invention to provide a power steering system of the foregoing character which is constructed so that an abnormality of the system can be recognized in a moment assuredly and is therefore desirable from a safety driving point of view.
It is a further object of the present invention to provide a power steering system of the foregoing character which can be obtained without requiring any expensive electromagnetic valve and therefore can be lower in cost.
To accomplish the above objects, there is provided according to an aspect of the present invention a power steering system for a vehicle comprising a hydraulic pressure source, a hydraulic cylinder having first and second chambers and driven in accordance with a pressure differential between a pressure fluid supplied to the first chamber and that to the second chamber, first and second communication conduits connecting the first and second chambers to the hydraulic pressure source, respectively, a controller for selectively switching a supply direction of the pressure fluid from the hydraulic pressure source to the first and second chambers of the hydraulic cylinder in response to a steering operation of a steering wheel, a bypass conduit connecting between the first and second communication conduits and having first and second bypass conduit ports, and first and second valves disposed in the bypass conduit for selectively opening and closing, in response to the pressure fluid from the pressure source, the first and second bypass conduit ports in such a manner that the pressure fluid from the hydraulic pressure source is supplied to the first chamber of the hydraulic cylinder while the pressure fluid in the second chamber of the hydraulic cylinder is discharged therefrom when the first valve is in a position of closing the first bypass conduit port and the second valve is in a position of opening the second bypass conduit port and the pressure fluid from the hydraulic pressure source is supplied to the second chamber of the hydraulic cylinder while the pressure fluid in the first chamber of the hydraulic cylinder is discharged therefrom when the first valve is in a position of opening the first bypass conduit port and the second valve is in a position of closing the second bypass conduit port, the first and second valves being placed in the positions of opening the first and second bypass conduit ports and thereby providing communication between the first and second communication conduits when the hydraulic pressure source is inoperative.
According to another aspect of the present invention, there is provided a power steering system for a vehicle comprising a double-acting hydraulic cylinder for controlling the direction of a pair of wheels of the vehicle, the hydraulic cylinder having first and second chambers and driven in accordance with a pressure differential between a pressure fluid supplied to the first chamber and that to the second chamber, and a hydraulic circuit for controlling supply and discharge of pressure fluid to and from the first and second chambers of the hydraulic cylinder, the hydraulic circuit including a hydraulic pressure source, a reservoir, and a pair of first and second valves, the first valve being disposed between the first chamber of the hydraulic cylinder and the reservoir for providing communication therebetween when open and thereby allowing discharge of the pressure fluid from the first chamber of the hydraulic cylinder and obstructing communication therebetween when closed and thereby allowing supply of the pressure fluid from the hydraulic pressure source to the first chamber, the second valve being disposed between the second chamber of the hydraulic cylinder and the reservoir for providing communication therebetween when open and thereby allowing discharge of the pressure fluid from the second chamber of the hydraulic cylinder and obstructing communication therebetween when closed and thereby allowing supply of the pressure fluid from the hydraulic pressure source to the second chamber of the hydraulic cylinder, the first and second valves being opened to communicate the first and second chambers of the hydraulic cylinder with the reservoir when the hydraulic pressure source is inoperative.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.